1. Field of the Invention
The present invention relates to a low-friction coating composition, and more particularly to a low friction coating composition, comprising an admixture of (1) a high temperature epoxy, (2) molybdenum disulfide powder, and (3) a polyimide powder. The present invention is also directed to a substrate coated with the low-friction coating composition of the invention, and a method of coating a substrate with the low-friction coating composition of the invention.
2. Brief Description of the Art
Low-friction coatings are useful in a wide variety of industrial applications to provide movable linkages or reciprocating parts with a low friction interface without the need for lubrication by grease, oil, or other lubricant. Low-friction coating compositions also provide high load capability with lower resistance than exhibited by metal/metal bearing contact, as well as having a relatively long life. In particular, low friction coatings have proven useful in aircraft and other industrial equipment, where severe operating conditions can occur. For example, aircraft fuel and oil pump mechanisms require repair because their gear drive bushings wear away during use. This wear (called "galling" or "scoring" or "gear wiping") limits the longevity of the pump mechanism and results in costly rebuilding, repair, or replacement of the moving components of the mechanisms. Low-friction coating compositions have also been used on reciprocating parts, such as engine pistons, where a low-friction interface between the reciprocating part and its housing is required for optimum performance.
Under extreme conditions, the wear resistance of a low-friction or self-lubricating coating composition becomes a key factor in how that coating composition performs in any particular application. Many low-friction or self-lubricating compositions available today do not have the wear resistance properties that are required for modern high-performance machinery parts made under close tolerances. Lack of wear-resistant properties in commercially available low friction coating compositions frequently results in premature degradation of the low-friction or self-lubricating coating. Without a wear resistant low friction coating, moving parts and/or linkages suffer accelerated wear which can result in costly replacement or complete failure of the parts. Therefore, efforts have been made to improve the wear resistance of low-friction or self-lubricating coatings.
U.S. Pat. No. 4,996,085 discloses coating a pump housing surface with a composition comprising a hardenable epoxy resin, a reinforcing filler, and an agent having a lower coefficient of friction than the epoxy resin. Graphite may function as the reinforcing filler, and the frictional reducing agent may be either graphite, in fibrous or granulated form; fluorinated carbon (i.e., --(CF.sub.x)--wherein x&lt;2); or molybdenum disulfide. Further, the preferred class of epoxy resin is a diglycidyl ether of a dihydric phenol (e.g., the diglycidyl ether of bisphenol A).
U.S. Pat. No. 5,316,790 discloses a lubricative coating and filler material for restoring metal surfaces that have become scratched, scored, grooved, or otherwise damaged to a functional condition. This coating and filler material is a mixture of an epoxy resin, tungsten disulfide, and isopropyl alcohol in a ratio of 9:1:1.6.
U.S. Pat. No. 5,554,020 to Rao et al. discloses a high efficiency pump having relatively-moving parts constituted of a light weight material and a coating on at least one of the parts. The coating is comprised of solid lubricants in a polymer resin matrix stable up to 700.degree. F. The solid lubricant may be graphite, molybdenum disulphide, boron nitride, tungsten disulphide, or polytetrafluoroethylene (PTFE). The resin matrix may be one of polyimides, epoxy, or polyaryl sulphone.
U.S. Ser. No. 09/183,859 discloses a curable composition useful as a self-lubricating coating and comprising an admixture of:
(1) 40-60 wt % of a first one-component epoxy resin composition, comprising: PA1 (2) 5-20 wt % of a second one-component epoxy resin composition, comprising: PA1 (3) 2-30 wt % of di-epoxide reactive diluent; PA1 (4) 2-20 wt % polytetrafluoroethylene; PA1 (5) 2-20 wt % molybdenum disulfide; and PA1 (6) 0.5-13 wt % mica. PA1 A. about 60 wt % to about 95 wt % of a high temperature epoxy, comprising PA1 B. about 1 wt % to about 30 wt % of molybdenum disulfide; and PA1 C. about 1 wt % to about 20 wt % of polyimide powder.
(a) about 10-30% by weight of 4-gylcidyloxy, n,n-diglycidyl aniline; PA2 (b) about 30-60% by weight of an epoxy resin (an epichlorohydrin ether of bisphenol A); and PA2 (c) about 3-7% by weight of strontium chromate; PA2 (a) about 50% to about 90% by weight of an epoxy resin (a diglycidyl ether of bisphenol A); PA2 (b) about 5% to about 25% by weight of diethylene glycol monoethyl ether acetate; and PA2 (c) about 5% to about 25% by weight of p-t-butylphenyl glycidyl ether; PA2 1. about 30 wt % to about 60 wt % of N,N'-m-phenylene dimaleimide; PA2 2. about 30 wt % to about 60 wt % of bisphenol epoxy resin; and PA2 3. about 1 wt % to about 5 wt % of amorphous silicon dioxide;
The preferred self-lubricating composition disclosed by U.S. Ser. No. 09/183,859 comprises: (1) about 40% to about 60% by weight of HYSOL EA 929NA epoxy resin composition; (2) about 5% to about 20% by weight HYSOL SR1000 high solids epoxy resin/solvent mixture; (3) about 2% to about 30% by weight of ARALDITE RD-2 di-epoxide reactive diluent; (4) about 2% to about 20% of ZONYL MP1000 polytetrafluoroethylene powder; (5) about 2% to about 20% by weight molybdenum disulfide powder; and (6) and about 0.5% to about 13% H360 wet ground mica.
Copending U.S. Ser. No. 08/568,114 discloses a curable composition useful for repairing worn surfaces on housings comprising an admixture of (1) a mixture of (a) at least one one-component epoxy resin, (b) at least one solvent and (c) at least one reactive diluent, wherein the epoxy resin is present in a major (i.e., at least 50% by weight) amount in the mixture; (2) graphite powder; and (3) polytetrafluoroethylene powder.
While these coating materials may be satisfactory for certain applications, there is still need for improved low-friction coating compositions that have high wear resistance, and that are useful for a wide variety of applications including pump housings and mechanisms, pistons, or other types of reciprocating parts where high wear resistance is required. The present invention is believed to be an answer to that need.